Omnidirectional slot antenna

Abstract: There is provided a small-sized and simplified horizontally polarized antenna apparatus which forms an omnidirectional pattern in the horizontal plane. The radiation field in the horizontal plane becomes continuous and a horizontally polarized omnidirectional radiation pattern can be obtained in the horizontal plane by forming radiation slots at opposing positions on a grounded hollow body and exciting the slots out of phase.

Description: The present invention relates to a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane, and to a transponder provided with such an antenna apparatus. Figs. 1(a) and 1(b) schematically illustrate a configuration of a horizontal polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane explained in Chapter 12 of "VHF Antenna" written by Uchida and Mushiake, and issued by the Production Technology Center (March, 1977). Fig. 1(a) is a perspective view and Fig. 1(b) is a top plan view with electric field distribution indicated by arrows. In these figures, the numeral 50 designates a dipole antenna and the symbol I indicates a current flowing through the dipole. Next, operations will be explained. A grounded conductor 51 includes four surfaces and a dipole antenna 50 is arranged at each surface. The dipole antenna 50 is arranged in parallel to the horizontal surface to excite a horizontally polarized wave. A plurality of dipole antennas may be arranged in the vertical direction. Amplitudes of currents flowing through the dipole antennas in the same height are equal, but phases thereof are sequentially different by 90 degrees. A dipole antenna 50 has a figure-8 type radiation directivity, but substantially horizontally polarized omnidirectivity can be obtained through a combination of the four dipole elements. Figs. 2(a) - 2(c) show a conventional slot antenna indicated in "X-band omnidirectional double-slot array antenna" by T. Takeshima, ELECTRONIC ENGINEERING, No. 39, pp. 617-621 (October, 1967). These figures schematically illustrate a configuration of a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane (rectangular waveguide slot antenna). Fig. 2(a) is a perspective view, Fig. 2(b) is a sectional view along the line A-A and Fig. 2(c) is a side elevation. In Figs. 2(a) - 2(c), numeral 60 designates a radiation slot; 61 a waveguide; and 62 a flange. The principle in operation of the rectangular waveguide slot antenna shown in Figs. 2(a) - 2(c) will be explained with reference to Figs. 3(a) and 3(b). Fig. 3(a) is a diagram illustrating a distribution of magnetic field inside the waveguide 61. Fig. 3(b) is a cross-sectional view along the line A-A illustrating a distribution of magnetic field inside the waveguide and a current flowing along the side surface. Such distributions of magnetic field and current as illustrated in Figs. 3(a) and 3(b) can be realized by short-circuiting the end portions of the waveguide. Electromagnetic waves propagated along the rectangular waveguide 61 excite the radiation slots 60 to radiate electromagnetic waves if the radiation slots 60 are provided in parallel with the waveguide axis at the positions offset from the center of the H plane of the rectangular waveguide 61. In this case, the radiation slots 60 are excited by providing each of the radiation slots 60 at a position where the magnetic field inside the waveguide 61 becomes maximum. An amount of electromagnetic wave radiation can be adjusted by changing the position of each radiation slot 60. In order that the waveguide slot antenna shown in Figs. 2(a) - 2(c) may be used as a horizontally polarized omnidirectional antenna, the radiation slots 60 are provided, as shown in Fig. 4(a), on the front and rear H planes of the waveguide 61. Then, a distribution of electric field in the horizontal plane changes as shown in Fig. 4(b). The radiation slots 60 are excited out of phase and the radiation field becomes continuous in the horizontal plane. As a result, a theoretically omnidirectional directivity can be realized. However, if, as shown in Fig. 2(a), two radiation slots are formed symmetrically on the front and rear surfaces, two radiation slots can be excited in the same phase by arranging the radiation slots in symmetrical positions of the waveguide 61 with respect to the center thereof at an interval of λg/2 (λg is a wavelength in the waveguide). Therefore, a vertically symmetrical pattern can be obtained in the direction of  = ±90° (in Fig. 4(a)), while a beam tilt is generated in the direction of  = 90° + α and  = 0° and 180° in Fig. 4(a) due to an array factor of the radiation field of the two radiation slots. Accordingly, on the x-y plane, a gain difference is generated in the direction of  = ±90°, 0° and 180° and a ripple in the horizontal plane becomes significant whereby no omnidirectivity can be achieved. In the case where one radiation slot is provided in a position offset from the center of the H plane of the waveguide, no symmetrical configuration is not formed and actually no omnidirectivity can be obtained. Fig. 5 schematically illustrates a configuration of a transponder 70 provided with an antenna 71 shown in Fig. 2(a). This transponder 70 is provided with a transmitter/receiver (transceiver) 72 connected to the horizontally polarized antenna 71 which has an omnidirectional pattern in the horizontal plane. In an emergency such as an accident, the transceiver 72 is activated by turning a switch 73 ON, getting the transceiver ready for receiving a signal. When the transceiver 72 under this condition receives a radar signal radiated from a searching plane, the transceiver 72 is switched to an electromagnetic wave radiation mode and transmits a response signal. Thus, a person who has met with an accident can inform his position by generating an emergency signal and await rescue by a searching plane. The transceiver 72 is connected to a battery 74 and the transponder 70 is covered with a radome 75. An existing horizontally omnidirectional antenna structured such as explained above is widely used as an antenna apparatus for TV and radar. However, if a dipole antenna as shown in Fig. 1(a) is used, the apparatus itself has protrusions having large volumes and there is a difficulty in fixing the antenna and wiring power supply cables. If a waveguide slot antenna as shown in Fig. 2(a) is used, a substantially omnidirectional pattern can easily be achieved by providing radiation slots on the waveguide, but, if a ripple in the horizontal plane becomes large, any omnidirectional pattern cannot be obtained. US 4, 247, 858 relates to an antenna for use with optical or high-frequency radiation. The antenna has a hollow conductor with at least one slot which acts as an input/output coupling opening. US 4, 590, 479, upon which the precharacterising portion of claim 1 is based discusses a hollow cylindrical slotted waveguide antenna which radiates a television signal with the aid of resonant radiating slots with elongated couplers adjacent to the slots and polarization insensitive couplers. Claim 6 has been limited upon the Takeshima publication. The present invention has been proposed to overcome the problems described above and it is therefore an object of the present invention to provide a small-sized horizontally polarized omnidirectional antenna having a simplified configuration. According to one aspect the present invention provides an antenna apparatus in accordance with claim 1. The cylindrical body can be excited in the TE 01 mode, whereby the radiation slot can be excited without using the conductive bar and an omnidirectional radiation pattern can be obtained. The conductive cylindrical body can be provided with a center conductor. This center conductor can be a spiral conductor. Since a current flows through the outer conductor at a slanting angle with respect to the longitudinal axis of the cylindrical body, the radiation slots provided along the longitudinal axis can be excited and an omnidirectional radiation pattern can be obtained in a plane perpendicular to the longitudinal axis. Horn-type conductive plates can be provided on the respective surfaces perpendicular to the longitudinal axis of the conductive cylindrical body. The horn-type conductive plates enable the beam width in a plane including the longitudinal axis to be reduced without changing size and position of the radiation slots and a high gain omnidirectional radiation pattern to be obtained in the plane perpendicular to the longitudinal axis. According to a further aspect the present invention provides an antenna apparatus in accordance with claim 6. In this aspect the hollow body is a rectangular waveguide having radiation slots formed on the center line of the H planes of the rectangular waveguide and a member for disturbing a distribution of electromagnetic field inside the rectangular waveguide. The member can comprise conductive bars fixed to one side edge of a corresponding radiation slot or can be a dielectric material mounted at a position deviated from the center line of the rectangular waveguide. The conductive bars and the dielectric material operate to distribute an electromagnetic field in the rectangular waveguide asymmetrically with respect to the center line, whereby the radiation slots provided on the center line of the H planes are excited and an omnidirectional radiation pattern having no beam tilt can be obtained. Meanwhile, it is also possible to excite the rectangular waveguide in the TE 20 mode, in place of providing the above electromagnetic field disturbing member in the rectangular waveguide. Since the electric field becomes zero along the center line of the H plane, the radiation slots provided on the center line of the H planes can be excited out of phase and thereby an omnidirectional radiation pattern can be obtained in a plane perpendicular to the longitudinal axis of the rectangular waveguide. The above and further objects and features of the present invention will be more clearly understood from a consideration of the following description taken in connection with the accompanying drawings. Fig. 1(a) is a perspective view of a conventional omnidirectional antenna apparatus. Fig. 1(b) is a plan view of the antenna apparatus of Fig. 1(a), illustrating a distribution of electric field. Fig. 2(a) is a perspective view illustrating another conventional omnidirectional antenna apparatus. Fig. 2(b) is a cross-sectional view taken along the line A-A of Fig. 2(a). Fig. 2(c) is a side elevation of the antenna apparatus of Fig. 2(a). Fig. 3(a) illustrates a distribution of magnetic field in the antenna apparatus of Fig. 2(a). Fig. 3(b) illustrates directions of current and magnetic field at the cross-section taken along the line A-A of Fig. 3(a). Fig. 4(a) is a diagram for explaining directivity of the antenna apparatus of Fig. 2(a). Fig. 4(b) illustrates a horizontal distribution of electric field established by the antenna apparatus of Fig. 4(a). Fig. 5 is a partially cutout diagram illustrating a conventional transponder. Fig. 6(a) is a perspective view of a first embodiment of an antenna apparatus of the present invention. Fig. 6(b) is a cross-sectional view taken along the line A-A of Fig. 6(a). Fig 6(c) is a side elevation of the antenna apparatus of Fig. 6(a). Fig. 7(a) is a perspective view of a second embodiment of an antenna apparatus of the present invention. Fig. 7(b) is a cross-sectional view taken along the line A-A of Fig. 7(a). Fig 7(c) is a side elevation of the antenna apparatus of Fig. 7(a). Fig. 8(a) is a perspective view of an antenna apparatus to explain TE 01 mode excitation. Fig. 8(b) illustrates a distribution of electromagnetic field at the cross-section taken along the line A-A of Fig. 8(a). Fig. 8(c) illustrates a distribution of a current on the side surface of the antenna apparatus of Fig. 8(a). Fig. 9 (a) is a perspective view of a third embodiment of an antenna apparatus of the present invention. Fig. 9(b) is a cross-sectional view taken along the line A-A of Fig. 9(a). Fig. 9(c) is a side elevation of the antenna apparatus of Fig. 9(a). Fig. 10(a) is a perspective view of a fourth embodiment of an antenna apparatus of the present invention. Fig. 10(b) is a cross-sectional view taken along the line A-A of Fig. 10(a). Fig. 10(c) illustrates a distribution of a current at the cross-section taken along the line A-A of Fig. 10(a). Fig. 11(a) is a perspective view of an antenna apparatus to explain excitation in the TE 20 mode. Fig. 11(b) illustrates a distribution of electric field at the cross-section taken along the line A-A of Fig. 11(a). Fig. 12 is a perspective view of an antenna apparatus with a spiral inner conductor. Fig. 13 is a perspective view of a radome usable with the antenna apparatus of the present invention. Embodiment 1 Figs. 6(a)-6(c) schematically illustrate a configuration of a first embodiment of the present invention, Fig. 6(a) being a perspective view, Fig. 6(b) a cross section taken along the line A-A and Fig. 6(c) a side elevation. In these figures, the radiation slots 1, 1' are formed to oppose each other on a cylindrical waveguide 17 of which both ends are short-circuited. To one side edge of each of the radiation slots 1, 1' are soldered conductive bars 18, 18'. Numberal 19 designates a waveguide flange. When the circular waveguide 17 is excited in a TM 01 mode, a current flows in the axial direction. If the radiation slots 1, 1' are provided in parallel to the axis of the waveguide 17, the radiation slots 1, 1' are not excited because the slots do not cross the current. The radiation slots 1, 1' can be excited by fixing the conductive bars 18, 18' inside the circular waveguide 17 from the side edges of the radiation slots 1, 1'. A horizontally polarised omnidirectional radiation pattern can be obtained by arranging one or more radiation slots in the circumferential direction of the cylindrical waveguide 17. The beam width in the vertical plane can be narrowed by arranging a plurality of radiation slots in parallel to the longitudinal axis of the circular waveguide 17. Since the radiation lots 1, 1' are excited by exciting the cylindrical conductor 17, a standing wave position deviates when an excitation frequency of the waveguide 17 changes. Then, the amplitude and phase of a signal exciting the radiation slots 1, 1' change and a radiation pattern obtained by combining radiation fields from the slots 1, 1' also changes. It is possible to provide the horn-type conductors 15, 15' to both ends of the circular waveguide 17 in order to obtain a narrower beam width in the vertical plane. The horn-type conductors 15, 15' operate in combination like a horn antenna. Since the gain of this antenna is determined by a size of the aperture of the horn, a higher gain can be obtained by enlarging the aperture of the horn. This means that a high gain can be obtained even if only one radiation slot is provided on each of the conductive plates 2, 2'. A slant angle α of the horn-type conductors 15, 15' with respect to the horizontal plane does not give any influence on an omnidirectional pattern in the horizontal plane. The beam width and gain in the vertical plane can be easily adjusted by changing the slant angle α. In this embodiment, the radiation slots 1, 1' are excited using the conductor bars 18, 18', but it is possible to excite the radiation slots 1, 1' by slanting the radiation slots 1, 1' with respect to the axis of the circular waveguide 17. Embodiment 2 Figs. 7(a) - 7(c) schematically illustrate a configuration of the second embodiment of the present invention, Fig. 7(a) being a perspective view, Fig. 7(b) a plan view taken along the line A-A and Fig. 7(c) a side elevation. In this embodiment a center conductor 20 is provided through the circular waveguide 17 of the first embodiment to form a coaxial line 17'. If the coaxial line 17' including the short-circuited ends is excited in the basic mode (the magnetic field is uniform in the circumferential direction of the coaxial line 17'), a current flows in the longitudinal axial direction. If the radiation slots 1, 1' are provided in parallel to the axis of the coaxial line 17', the radiation slots 1, 1' are not excited. In order that these slots are excited, the conductor bars 18, 18' are provided to protrude inside the coaxial line 17' from the side edges of the radiation slots 1, 1'. A horizontally polarized omnidirectional radiation pattern can be obtained by providing one or more radiation slots in the circumferential direction. In order to make the beam in the vertical direction narrower, a plurality of radiation slots may be arranged in parallel to the axis of the coaxial line 17'. Since the radiation slots 1, 1' are excited by exciting the coaxial line 17' the position of a standing wave is deviated if the excitation frequency of the coaxial line 17' is shifted. Then, the amplitude and phase of a signal for exciting the radiation slots 1, 1' change and a radiation pattern obtained by combining the radiation fields from the slots 1, 1' is also changed. In order to avoid this problem, the horn-type conductors 15, 15' may be provided, as in the case of the first embodiment, to both ends of the coaxial line 17' in view of obtaining a narrower beam width in the vertical direction. Figs. 8(a) - 8(c) schematically illustrate the effect of excitation in the TE 01 mode, Fig. 8(a) being a perspective view, Fig. 8(b) showing a distribution of electromagnetic wave at the cross-section taken along the line A-A and Fig. 8(c) showing a distribution of current on the side surface. The cylindrical waveguide 17 is excited in the TE 01 mode and the radiation slots 1, 1', 1", 1'" are formed in the axial direction of the cylindrical waveguide 17. In these figures, since the cylindrical waveguide 17 having the short-circuited ends is excited in the TE 01 mode, a current flows in the circumferential direction of the cylindrical waveguide 17 as shown in Fig. 8(c). Therefore, the radiation slots can easily be excited by providing the slots in parallel to the longitudinal axis of the waveguide. A horizontally polarized omnidirection radiation pattern can be obtained by arranging one or more slots in the circumferential direction. A beam width in the vertical direction can be narrowed by arranging a plurality of radiation slots in the longitudinal axial direction of the waveguide 17 or providing horn-type conductors at both ends of the circular waveguide 17. Embodiment 3 Figs. 9(a) - 9(c) schematically illustrate a configuration of the third embodiment of the present invention, Fig. 9(a) being a perspective view, Fig. 9(b) a cross-sectional view taken along the line A-A and Fig. 9(c) a side elevation. In this embodiment, the radiation slots 1, 1' are formed on two opposing surfaces of a rectangular waveguide 21. If the rectangular waveguide 21 having short-circuited ends is excited in the TE 01 mode, the radiation slots 1, 1' must be formed at positions offset from the longitudinal axis of the waveguide 21 for excitation. Then, a beam tilt is generated like in the prior art and a ripple in the horizontal plane becomes large. In this embodiment, the radiation slots 1, 1' are provided in parallel with the center line of the H plane of the rectangular waveguide 21 and the conductive bars 18, 18' protruding inside the waveguide 21 are fixed to the side edges of the radiation slots 1, 1'. The conductive bars 18, 18' establish a distribution of electromagnetic field asymmetrical with respect to the center line of the rectangular waveguide 21, whereby the radiation slots 1, 1' provided on the center line of the plane H are excited, resulting in the generation of an omnidirectional radiation pattern having no beam tilt. Embodiment 4 Figs. 10(a) - 10(c) schematically illustrate a configuration of the fourth embodiment of the present invention, Fig. 10(a) being a perspective view, Fig. 10(b) a cross-sectional view taken along the line A-A and Fig. 10(c) showing a distribution of electric field at the cross-section taken along the line A-A. In this embodiment, a dielectric material 22 is fixed inside the rectangular waveguide 21 in place of the conductive bars 18, 18' used in the third embodiment. If the rectangular waveguide 21 having short-circuited ends is excited in the TE 01 mode, the radiation slots 1, 1' must be formed at positions offset from the center of the waveguide 21 for the excitation. Then, a beam tilt is generated like in the prior arts and a ripple in the horizontal plane becomes large. In this embodiment, though the radiation slots 1, 1' are provided in parallel to the center line of the H plane of the rectangular waveguide 21, the dielectric material 22 is provided at the position offset from the center of the rectangular waveguide 21, whereby the radiation slots 1, 1' are excited as a result of a change in distribution of the electromagnetic field inside the rectangular waveguide 21 as shown in Fig. 10(c). Since the conductive bars 18, 18' are not used in this embodiment, such a process as soldering is advantageously unnecessary. Figs. 11(a) and 11(b) schematically illustrate excitation in the TE 20 mode, Fig. 11(a) being a perspective view and Fig. 11(b) showing a distribution of electric field at a cross-section taken along the line A-A. In this embodiment, the rectangular waveguide 21 is excited in the TE 20 mode and the ends of the rectangular waveguide 21 are short-circuited. As a result, the electromagnetic field inside the rectangular waveguide 21 becomes zero at the center of the H plane as shown in Fig. 11(b), whereby the radiation slots 1, 1' can be excited out of phase. The radiation field from the radiation slots 1, 1' becomes continuous in the horizontal plane and a horizontally polarized omnidirectional radiation pattern can be obtained. Fig. 12 schematically illustrates a configuration of antenna in which the center conductor of the second embodiment is made spiral. If the coaxial line 17', the ends of which are short-circuited, is excited in the basic mode (the magnetic field is uniform in the circumferential direction of the coaxial line 17'), a current flows in the longitudinal axial direction. If the radiation slots 1, 1', 1", 1"' are provided in parallel to the longitudinal axis of the line 17', the radiation slots are not excited. In the illustrated arrangement the spiral inner conductor 23 is used in place of the straight centre conductor 20 of the second embodiment. The spiral inner conductor 23 enables a current to flow through the outer conductor slantly with respect to the longitudinal axis, and the radiation slots 1, 1', 1", 1"' provided in parallel to the longitudinal axis can be excited. A horizontal polarization omnidirectional radiation pattern can be obtained by arranging one or more radiation slots in the circumferential direction of the coaxial line 17'. In order to reduce the beam width in the vertical plane, a plurality of radiation slots may be arranged in the longitudinal axial direction of the coaxial line 17' or horn-type conductors can be provided as explained above. The whole part or a part of the inner conductor 23 may be formed in spiral and the end of the inner conductor 23 may be open or short-circuited. Fig. 13 schematically illustrates a radome usable with the above embodiments of the present invention. In this figure, a radome 28 has radiation slots 29, 29', 29", ... and accommodates any one of the omnidirectional antennas 30 described in the foregoing embodiments. In general, if a radome is used to protect an antenna apparatus, the radiation pattern is influenced to a certain degree by the radome even if the radome is transparent to an electromagnetic wave. To solve this problem, the radome 28 comprises a cylindrical cover of a dielectric material and a conductive film formed on the inner surface of the cylindrical cover, radiation slots 29, 29', 29", ... being formed on the conductive film in order to reradiate the electromagnetic wave to obtain an omnidirectional radiation pattern. Since a plurality of radiation slots are provided in the circumferential direction of the radome 28, an omnidirectional radiation pattern can be obtained without any influence given by the radome 28. It is noted that, a plurality of radiation slots may be arranged along the longitudinal axis of the radome 28 and dipole antennas may be used in place of the slots.

Claims: An antenna apparatus having at least two radiation slots (1,1') arranged at diametrically opposed positions on a grounded conductive cylindrical hollow body (17), said radiation slots (1,1') being excited out of phase to form an omnidirectional radiation pattern in a plane perpendicular to said cylindrical hollow body (17); wherein said radiation slots (1,1') are formed along the longitudinal axis of said cylindrical hollow body (17) and respective conductive bars (18,18') are fixed inside of said cylindrical hollow body (17) to one side edge of each radiation slot (1,1') characterised in that , when viewed individually in an outward direction from a central axis of said cylindrical hollow body, said conductive bars are fixed to opposite side edges of said diametrically opposed radiation slots. An antenna apparatus recited in claim 1, wherein said cylindrical body (17) is excited in the TE01 mode. An antenna recited in claim 1 or 2, wherein said cylindrical conductive body (17) has a centre conductor (20, 23) therein. An antenna apparatus recited in claim 3, wherein said centre conductor is a spirally formed conductor (23). An antenna apparatus recited in claim 1, 2, 3 or 4 wherein horn-type conductive plates (15,15') are provided to the planes perpendicular to the longitudinal axis of said cylindrical conductive body (17). An antenna apparatus having at least two radiation slots (1,1') arranged at opposed positions on a grounded conductive hollow body (21), said radiation slots (1,1') being excited out of phase to form an omnidirectional radiation pattern in a plane perpendicular to said hollow body (21), wherein said radiation slots (1, 1') are formed along the longitudinal axis of said hollow body (17) said hollow body (21) being a rectangular waveguide characterised by having said radiation slots (1,1') formed on the centre line of the H planes of said rectangular waveguide (21) and by at least one member (18,18', 22) for disturbing a distribution of the electromagnetic field being provided inside said rectangular waveguide (21) fixed to a side edge of each radiation slots (1, 1'), wherein when viewed individually in an outward direction from a central axis of said hollow body (21), said at least one member is fixed to opposite side edges of said opposed radiation slots. An antenna apparatus recited in claim 6, wherein said at least one member (18,18') are conductive bars (18,18') each fixed to one side edge of a corresponding one of said radiation slots (1,1'). An antenna apparatus recited in claim 6, wherein said at least one member (22) is a dielectric material (22) mounted at a position deviated from the centre line of said rectangular waveguide (21). An antenna apparatus recited in claim 6, 7 or 8, wherein said rectangular waveguide (21) is excited in the TE20 mode.

Inventors: KATAGI TAKASHI, OHMINE HIROYUKI, SATO SHIN-ICHI, SUNAHARA YONEHIKO, WADAKA SHUSOU, KATAGI, TAKASHI, OHMINE, HIROYUKI, SATO, SHIN-ICHI, SUNAHARA, YONEHIKO, WADAKA, SHUSOU, Katagi, Takashi, c/o Mitsubishi Denki K.K., Ohmine, Hiroyuki, c/o Mitsubishi Denki K.K., Sato, Shin-Ichi, c/o Mitsubishi Denki K.K., Sunahara, Yonehiko, c/o Mitsubishi Denki K.K., Wadaka, Shusou, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1994
Main classification: H01Q13
IPC codes: H01Q21, H01Q13